Light scanning apparatus

ABSTRACT

A laser beam emitted from a laser light source is shaped into a substantially parallel beam in the main scanning direction, and is condensed in the vicinity of a deflecting surface of a deflector in the sub scanning direction by a light source optical system. The laser beam deflected by the deflector is projected onto a photosensitive body of an image forming apparatus by a scanning optical system. The light source optical system is constituted by one optical element made of resin. The optical element has a reflecting surface having no symmetry axis of rotation, and two transmitting surfaces. The laser beam incident on the incident side transmitting surface is reflected at the reflecting surface, and exits from the exit side transmitting surface. As described above, by providing the reflecting surface with a beam shaping function, the performance change caused when there is a temperature change can be reduced more than when a refracting surface is provided with the beam shaping function. The transmitting surfaces may be provided with a diffracting function.

[0001] The present application claims priority to Japanese PatentApplication No. 2001-86838 filed Mar. 26, 2001, the entire content ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a light scanning apparatus, forexample, to a laser scanner constituting a print head of an imageforming apparatus such as a laser printer or a digital copier.

[0004] 2. Description of the Related Art

[0005] In the field of laser scanners, conventionally, a technology hasbeen proposed to suppress an optical performance change due to arefractive index change or a configuration change caused by atemperature change, and a performance change due to a mode hop (that is,an oscillation wavelength change) of the laser diode by using adiffracting element as the light source optical system (for example,U.S. Pat. Nos. 6,067,106 and 6,101,020).

[0006] It is desirable that the light source optical system used for alaser scanner be made of resin for weight and size reduction. However,in the conventional laser scanners, when the light source optical systemis made of resin, the condition for compensating for the performancechange due to a temperature change and the condition for compensatingfor the performance change due to an oscillation wavelength change (thatis, axial chromatic aberration) significantly differ from each other, sothat performance cannot be maintained.

SUMMARY OF THE INVENTION

[0007] A principal object of the present invention is to provide a laserscanner being inexpensive and whose performance is stable even whenthere is a temperature change.

[0008] This and other objects are achieved by a laser scanner comprisinga laser light source emitting a laser beam; a deflector deflecting anincident laser beam in a main scanning direction; a light source opticalsystem constituted by one optical element made of resin and having: afirst transmitting surface on which the laser beam emitted from thelaser light source is incident; at least one reflecting surfacereflecting the laser beam incident on the first transmitting surface,and having no symmetry axis of rotation; and a second transmittingsurface from which the laser beam reflected by the reflecting surfaceexits, said light source optical system shaping the laser beam emittedfrom the laser light source into a substantially parallel beam in themain scanning direction, and condensing the laser beam in a vicinity ofa deflecting surface of the deflector in a sub scanning direction; and ascanning optical system again condensing the laser beam deflected by thedeflector.

[0009] The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view showing a laser scanner according tothe present invention;

[0011]FIG. 2 is a perspective view showing an optical elementconstituting a first embodiment (first example);

[0012]FIGS. 3A and 3B are optical structure views showing a crosssection in a main scanning direction and a cross section in a subscanning direction of the optical element constituting the firstembodiment (first example);

[0013]FIG. 4 is a plan view showing the groove configuration of adiffracting surface of the optical element constituting the firstexample;

[0014]FIG. 5 is a cross-sectional view showing the groove configurationof the diffracting surface of the optical element constituting the firstexample;

[0015]FIG. 6 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in the first example;

[0016]FIG. 7 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the first example;

[0017]FIG. 8 is a perspective view showing an optical elementconstituting a second embodiment (second example);

[0018]FIGS. 9A and 9B are optical structure views showing a crosssection in the main scanning direction and a cross section in the subscanning direction of the optical element constituting the secondembodiment (second example);

[0019]FIG. 10 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in the second example;

[0020]FIG. 11 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the second example;

[0021]FIG. 12 is a perspective view showing an optical elementconstituting a third embodiment (third example);

[0022]FIGS. 13A and 13B are optical structure views showing acrosssection in the main scanning direction and a cross section in the subscanning direction of the optical element constituting the thirdembodiment (third example);

[0023]FIG. 14 is a plan view showing the groove configuration of adiffracting surface of the optical element constituting the thirdexample;

[0024]FIG. 15 is a cross-sectional view showing the groove configurationof the diffracting surface of the optical element constituting the thirdexample;

[0025]FIG. 16 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in the third example;

[0026]FIG. 17 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the third example;

[0027]FIG. 18 is a perspective view showing an optical elementconstituting a fourth embodiment (fourth example);

[0028]FIGS. 19A and 19B are optical structure views showing a crosssection in the main scanning direction and a cross section in the subscanning direction of the optical element constituting the fourthembodiment (fourth example);

[0029]FIG. 20 is a plan view showing the groove configuration of adiffracting surface of the optical element constituting the fourthexample;

[0030]FIG. 21 is a cross-sectional view showing the groove configurationof the diffracting surface of the optical element constituting thefourth example;

[0031]FIG. 22 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in the fourth example;

[0032]FIG. 23 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the fourth example;

[0033]FIG. 24 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in a first comparativeexample;

[0034]FIG. 25 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the first comparativeexample;

[0035]FIG. 26 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in a second comparativeexample;

[0036]FIG. 27 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the second comparativeexample;

[0037]FIG. 28 is a graph showing a result of a simulation of imagesurface changes in the main scanning direction in a third comparativeexample; and

[0038]FIG. 29 is a graph showing a result of a simulation of imagesurface changes in the sub scanning direction in the third comparativeexample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Hereinafter, a laser scanner embodying the present invention willbe described with reference to the drawings. The same and correspondingparts of the embodiments are designated by the same reference numbers,and overlapping descriptions are omitted.

[0040]FIG. 1 shows a schematic structure of the laser scanner embodyingthe present invention. This laser scanner performs exposure scanning ona photosensitive body 6 with a laser beam 2 in an image formingapparatus such as a laser printer or a digital copier. The laser scannerincludes a laser light source 1, an optical element 3, a polygonalmirror 4 and a scanning optical system 5. First, the laser beam 2emitted from the laser light source 1 is incident on the optical element3. The optical element 3 is a light source optical system that shapesthe laser beam 2 by reflecting and refracting it. The optical element 3shapes the laser beam 2 emitted from the laser light source 1 into asubstantially parallel beam in the main scanning direction, andcondenses the laser beam 2 in the vicinity of a deflecting surface 4 aof the polygonal mirror 4 in the sub scanning direction. The laser beam2 shaped by the optical element 3 is incident on the polygonal mirror 4.The polygonal mirror 4 reflects the incident laser beam 2 at adeflecting surface 4 a to thereby deflect the beam 2 in the mainscanning direction. The laser beam 2 deflected by the polygonal mirror 4is reflected and refracted by the scanning optical system 5, and iscondensed into a spot on the photosensitive body 6. The main scanningdirection is a direction in which the laser beam 2 scans thephotosensitive body 6. The sub scanning direction is a directionvertical to the main scanning direction.

[0041] The laser scanner according to the present invention features thelight source optical system constituted by one optical element 3. Thebasic structures of the other optical members (FIG. 1) are common to allthe embodiments. FIGS. 2, 3A and 3B show a schematic optical structureof the optical element 3 of a first embodiment. FIGS. 8, 9A and 9A showa schematic optical structure of the optical element 3 of a secondembodiment. FIGS. 12, 13A and 13B show a schematic optical structure ofthe optical element 3 of a third embodiment. FIGS. 18, 19A and 19B showa schematic optical structure of the optical element 3 of a fourthembodiment. FIGS. 2, 8, 12 and 18 show the optical elements 3 and theoptical paths viewed from a slanting direction. FIGS. 3A, 9A, 13A and19A show the optical surfaces and the optical paths on a cross sectionin the main scanning direction. FIGS. 3B, 9B, 13B and 19B show theoptical surfaces and the optical paths on a cross section in the subscanning direction. In the optical structure views, the surfaces markedwith Si(i=1,2, . . . ) are the it-th surfaces counted from the side ofthe laser light source 1 along the optical paths. The surfaces markedwith Si followed by $ are surfaces having no symmetry axis of rotation(in other words, free-form surfaces). The surfaces marked with Sifollowed by are diffracting surfaces.

[0042] In the embodiments, the optical element 3 is made of resin, andhas four optical surfaces in the first to third embodiments and threeoptical surfaces in the fourth embodiment. Of the optical surfaces, atleast one surface is a reflecting surface having no symmetry axis ofrotation and two surfaces are transmitting surfaces. One of thetransmitting surfaces is a transmitting surface (Si) on the lightincident side (that is, the side of the laser light source 1), and theother transmitting surface is a transmitting surface (S4 or S3) on thelight exit side. In all the embodiments, since the optical element 3 ismade of resin, weight and cost reduction is achieved, and since thereflecting surface of the optical element 3 is structured so as tototally reflect light, it is unnecessary to apply a reflection coatingto the reflecting surface. Cost reduction is achieved also by this.

[0043] In the first embodiment, the light incident side transmittingsurface (S1) and the two reflecting surfaces (S2, S3) are surfaceshaving no symmetry axis of rotation, and the light exit sidetransmitting surface (S4) is a diffracting surface having a structuresuch that fine grooves are formed on a plane surface. The grooves areelliptically formed, and arranged so that the axial principal ray passesthrough the center of the ellipses. By elliptically forming the grooveson the diffracting surface, a diffractive power optimized to differentintensities in the main and sub scanning directions is obtained. In thesecond embodiment, the two reflecting surfaces (S2, S3) are surfaceshaving no symmetry axis of rotation, and the transmitting surfaces (S1,S4) are both plane surfaces that are not diffracting surfaces.

[0044] In the third embodiment, the two reflecting surfaces (S2, S3) aresurfaces having no symmetry axis of rotation, and the transmittingsurfaces (S1, S4) are both diffracting surfaces. The diffracting surfaceconstituting the light incident side transmitting surface (S1) is anaxisymmetric diffracting surface having a structure such that groovesare formed on a conical surface. The symmetry axis of rotation of theaxisymmetric diffracting surface is outside the area of the opticalsurface. The diffracting surface constituting the light exit sidetransmitting surface (S4) has a structure such that grooves are formedon a plane surface. In the fourth embodiment, the light incident sidetransmitting surface (S1) and one reflecting surface (S2) are surfaceshaving no symmetry axis of rotation, and the light exit sidetransmitting surface (S3) is an axisymmetric diffracting surface havinga structure such that fine grooves are formed on a plane surface. Thegrooves are concentrically formed, and arranged so that the axialprincipal ray passes through the center of the concentric circles. Sincethe number of reflecting surfaces is small and the diffracting surfacestructure is simplified, the fourth embodiment can be realized at alower cost than the other embodiments.

[0045] When the optical element 3 is made of resin, the image surfaceshift caused by a temperature change is large compared to when theoptical element 3 is made of glass. This is because the configurationchange (expansion, shrinkage or the like) and the refractive indexchange caused by a temperature change are larger in resin than in glass.In the laser scanners of the embodiments, a predetermined beam shapingfunction (a collimator lens function, a cylinder lens function or thelike) is provided by the light source optical system being constitutedby one optical element 3 made of resin and the optical element 3 beingprovided with at least one reflecting surface which is not a planesurface. Although a performance change due to the configuration changeis caused on the reflecting surface, no refractive index change thataffects the performance is caused. Therefore, the performance changecaused when there is a temperature change can be reduced even though thelight source optical system is made of resin. Further, the optical powercan be set to be weak even though no diffracting surface is used like inthe second embodiment or one or two diffracting surfaces are used likein the first, third and fourth embodiments. As described below, it canbe prevented that a large performance change is caused when there is awavelength change.

[0046] In the first, third and fourth embodiments, at least one of thetransmitting surfaces is a diffracting surface, and the provision of thediffracting surface suppresses the image surface shift caused by atemperature shift. This uses a fact that the oscillation wavelength ofthe laser diode increases when the temperature rises. The laser diodeused as the laser light source 1 has a characteristic such that theoscillation wavelength increases as the temperature rises. Diffractingsurfaces are sensitive to wavelength changes, and when the wavelength ischanged by a temperature change, the workings of diffracting surfacesare also changed. In the first, third and fourth embodiments, theoptical power of the diffracting surface and the like are set so thatthe direction in which the image surface is shifted by a wavelengthincrease and the direction in which the image surface is shifted by atemperature rise are opposite to each other. By this, an image surfacechange caused when there is a temperature change can be suppressedtogether with an image shift caused when there is a configurationchange.

[0047] In the third embodiment, the transmitting surfaces (S1, S4) areboth diffracting surfaces, and the directions of diffraction anglechanges on the two diffracting surfaces (S1, S4) caused when there is awavelength change are opposite to each other. While the transmittingsurfaces (S1, S4) are both inclined with respect to the sub scanningdirection in the schematic structure as shown in FIG. 13B, the opticalelement 3 is designed so that the principal ray of the laser beam 2travels substantially in a straight line by diffraction. That is, as aresult of the refractive power on each of the transmitting surfaces (S1,S4) being canceled out by the optical power of the diffracting surface,the principal ray obliquely incident on the transmitting surfaces (S1,S4) passes through the transmitting surfaces (S1, S4) in a straight linewithout its direction changed. In a case where the optical element 3 isdesigned so that the principal ray passes through the inclineddiffracting surfaces in a straight line as described above, when thewavelength changes, the direction of diffraction also changes, so thatthe position of beam condensation shifts. However, by designing theoptical element 3 so that the directions of diffraction angle changescaused when there is a wavelength change are opposite to each other likein the third embodiment, the change of the working of the diffractingsurface is canceled out when the wavelength changes, so that theposition of beam condensation does not shift in the optical element 3 asa whole.

EXAMPLES

[0048] Hereinafter, the laser scanner embodying the present inventionwill more concretely be described with reference to construction data ofthe optical element 3 constituting the laser scanner. First to fourthexamples shown below correspond to the above-described first to fourthembodiments. The optical structure views (FIGS. 2, 3A, 3B, 8, 9A, 9B,12, 13A, 13B, 18, 19A and 19B) of the optical elements 3 in the first tofourth examples show the optical structures of the optical elements 3 ofthe corresponding first to fourth examples.

[0049] TABLEs 1, 6, 9 and 16 show coordinate data of the opticalsurfaces of the optical elements 3 constituting the first to fourthexamples. These coordinate data show the origins of local coordinatesystems (x, y, z) in global coordinate systems (X, Y, Z), and thedisposition of the optical surfaces by vectors. TABLEs 2 to 4, 7, 8, 10to 13, 17 and 18 show configuration data of the optical surfaces of theoptical elements 3 constituting the first to fourth examples. Theconfigurations of the free-form surfaces are expressed by the expression($1) shown below, and the configurations of the axisymmetric diffractingsurfaces are expressed by the expression ($2) shown below. TABLEs 5, 14,15 and 19 show coefficients of phase functions p of the diffractingsurfaces used for the optical elements 3 of the first, third and fourthexamples. The phase functions p of the diffracting surfaces areexpressed by the expression (#1) shown below, and the phase functions pof the axisymmetric diffracting surfaces are expressed by the expression(#2) shown below. $\begin{matrix}{x = {\sum\limits_{i = 0}^{10}{\sum\limits_{j = 0}^{6}{a_{ij}y^{i}z^{j}}}}} & ({\$ 1}) \\{x = {\sum\limits_{i = 0}^{1}{a_{i}\left( \sqrt{y^{2} + z^{2}} \right)}^{i}}} & ({\$ 2}) \\{p = {\sum\limits_{i = 0}^{2}{\sum\limits_{j = 0}^{2}{b_{ij}y^{i}z^{j}}}}} & ({\# 1}) \\{p = {\sum\limits_{i = 0}^{2}{b_{i}\left( \sqrt{y^{2} + z^{2}} \right)}^{i}}} & ({\# 2})\end{matrix}$

[0050]FIG. 4 shows the groove configuration of the diffracting surfaceof the optical element 3 constituting the first example. For ease ofview, five grooves are shown as one groove. The grooves are ellipticallyformed, and have different diffractive powers in the main scanningdirection and in the sub scanning direction. FIG. 5 shows thecross-sectional configuration (cross section including the surfacenormal) of the diffracting surface of the optical element 3 constitutingthe first example. For ease of view, the cross section is magnified inthe vertical direction of FIG. 5 (the direction of the surface normal)by 100 times in length-to-width ratio. As shown in FIG. 5, the surfaceis constituted by an aggregation of curved surfaces. The curved surfacesare surfaces having no symmetry axis of rotation.

[0051]FIGS. 14A and 14B show the groove configurations of thediffracting surfaces of the optical element 3 constituting the thirdexample. FIG. 14A shows the groove configuration of the axisymmetricdiffracting surface formed on the transmitting surface (S1). FIG. 14Bshows the groove configuration of the diffracting surface formed on thetransmitting surface (S4). For ease of view, ten grooves are shown asone groove. On the transmitting surface (S1), the grooves formed on theconical surface are arcs whose center is the same as the center of thesymmetry axis of rotation of the cone. On the transmitting surface (S4),the grooves formed on the plane surface are parallel straight lines. Thegrooves are not uniformly spaced on both of the diffracting surfaces(S1, S4). FIGS. 15A and 15B show the cross-sectional configuration(cross section including the surface normal) of the diffracting surfaceof the optical element 3 constituting the third example. FIG. 15A showsthe groove configuration of the axisymmetric diffracting surface formedon the transmitting surface (S1). FIG. 15B shows the grooveconfiguration of the diffracting surface formed on the transmittingsurface (S4). As shown in FIGS. 15A and 15B, the surfaces are eachconstituted by an aggregation of plane surfaces. On the diffractingsurface (S1) formed on the conical surface, the parts of the standingwalls are cylindrical surfaces although the surfaces where light passesare plane surfaces.

[0052] In the third example, although the two diffracting surfaces (S1,S4) are both inclined in the schematic structure as mentioned above, theprincipal ray travels substantially in a straight line by diffraction.At this time, although the directions of diffraction change when thewavelength changes, by canceling out the direction change by the twodiffracting surfaces (S1, S4), the position of beam condensation doesnot shift due to the wavelength change in the optical element 3 as awhole.

[0053]FIG. 20 shows the groove configuration of the diffracting surfaceof the optical element 3 constituting the fourth embodiment. For ease ofview, five grooves are shown as one groove. Since the grooves are formedconcentrically, the diffractive power is the same in the main and subscanning directions. FIG. 21 shows the cross-sectional configuration(cross section including the surface normal) of the diffracting surfaceof the optical element 3 constituting the fourth example. For ease ofview, the cross section is magnified in the vertical direction of FIG.21 (the direction of the surface normal) by 100 times in length-to-widthratio. As shown in FIG. 21, the surface is constituted by an aggregationof curved surfaces.

[0054]FIGS. 6, 7, 10, 11, 16, 17, 22, 23, 24, 25, 26, 27, 28 and 29 showresults of simulations of image surface changes (mm) caused when thereis a temperature or wavelength change in the first to fourth examplesand first to third comparative examples (the horizontal axis representsthe image height in millimeters in the main scanning direction). FIGS.6, 10, 16, 22, 24, 26 and 28 show image surface changes in the mainscanning direction in the first to fourth examples and the first tothird comparative examples. FIGS. 7, 11, 17, 23, 25, 27 and 29 showimage surface changes in the sub scanning direction in the first tofourth examples and the first to third comparative examples. The linesplotted by ♦ are image surface changes caused when the temperature risesby 10 degrees. The lines plotted by ▪ show image surface changes causedwhen the wavelength increases by 2 nm. The lines plotted by Δ show imagesurface changes caused when the temperature rises by 10 degrees and thewavelength does not change for comparison.

[0055] Since almost all of the image surface changes caused when thereis a wavelength change are due to the workings of the diffractingsurfaces, the lines plotted by Δ can be considered substantially thesame as the lines obtained when no diffracting surface is used. Thechange in the oscillation wavelength of the laser diode caused when thetemperature rises by 10 degrees is approximately 2 nm. Therefore, thesum of the image surface change (Δ) caused when the temperature rises by10 degrees and the wavelength does not change and the image surfacechange (▪) caused when the wavelength increases by 2 nm is substantiallythe same as the image surface change (♦) caused when the temperaturerises by 10 degrees.

[0056] In the first to third comparative examples, an axisymmetriccollimator lens and cylinder lens are used instead of the opticalelement 3 in the specifications similar to those of the embodiments.However, in the first comparative example, the collimator lens and thecylinder lens are both made of glass, and in the second and thirdcomparative examples, the collimator lens and the cylinder lens are bothmade of resin. Although the second comparative example uses nodiffracting surface, the third comparative example uses one diffractingsurface for each of the collimator lens and the cylinder lens to therebycontrol the performance change when there is a temperature change. Thefirst comparative example is excellent in performance but is high incost compared to the first to fourth examples because it is made ofglass. The second comparative example is inexpensive because it is madeof resin, but is inferior in performance to the first to fourth examplesbecause it uses no reflecting surface. The third comparative example isinexpensive because it is made of resin, and the performance changecaused when there is a temperature change is sufficiently suppressed.However, the performance change caused when there is a wavelength changeis large.

[0057] As described above, according to the present embodiments, sincenot only the light source optical system is constituted by one opticalelement made of resin but also the optical element is provided with atleast one reflecting surface having no symmetry axis of rotation, thelaser scanner can be reduced in cost and the performance change causedby temperature and wavelength changes can be reduced.

[0058] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the present invention, they should be construedas being included therein. TABLE 1 Example 1 . . . coordinate data ofthe optical surfaces origins of y vector of local local coordinate xvector of local coordinate systems coordinate systems systems Surface XY Z X Y Z X Y, Z S1 free-formed 0.00 53.77 0.00 0.0000 −1.0000 0.00001.0000 0.0000 S2 free-formed 0.00 50.65 0.00 0.0000 −0.6428 0.76601.0000 0.0000 S3 free-formed 0.00 49.77 −5.00 0.0000 0.6428 −0.76601.0000 0.0000 S4 free-formed 0.00 44.77 −5.00 0.0000 −1.0000 0.00001.0000 0.0000

[0059] TABLE 2 Example 1 . . . configuration data a_(ij) of the firstsurface (S1) j i 0 1 2 3 4 0 0.00000 0.00000 2.70489 * 10⁻² −2.46735 *10⁻³ 4.93644 * 10⁻⁴ 2 9.65977 * 10⁻³   3.51036 * 10⁻³ 1.69484 * 10⁻⁴  1.41582 * 10⁻⁴ 0.00000 4 7.63719 * 10⁻⁴ −6.41266 * 10⁻⁶ 0.000000.00000 0.00000

[0060] TABLE 3 Example 1 . . . configuration data a_(ij) of the secondsurface (S2) j i 0 1 2 3 4 5 6 0 0.00000 0.00000   1.81264 * 10⁻²−4.95934 * 10⁻⁴   5.85072 * 10⁻⁵ −4.50839 * 10⁻⁶ 3.91027 * 10⁻⁷ 2−3.49770 * 10⁻⁴   8.24262 * 10⁻⁴ −8.13281 * 10⁻⁵   8.35928 * 10⁻⁶−6.17103 * 10⁻⁷ 0.00000 0.00000 4   1.51558 * 10⁻⁵ −3.07229 * 10⁻⁶  6.09714 * 10⁻⁹ −6.20261 * 10⁻⁷ 0.00000 0.00000 0.00000 6 −1.10212 *10⁻⁶ −1.94448 * 10⁻⁷   2.19668 * 10⁻⁸ 0.00000 0.00000 0.00000 0.00000 8  1.93562 * 10⁻⁷   8.10479 * 10⁻⁹ 0.00000 0.00000 0.00000 0.000000.00000 10 −8.39267 * 10⁻⁹ 0.00000 0.00000 0.00000 0.00000 0.000000.00000

[0061] TABLE 4 Example 1 . . . configuration data a_(ij) of the thirdsurface (S3) j i 0 1 2 3 4 5 6 0 0.00000 0.00000 −1.49589 * 10⁻²  1.91755 * 10⁻⁴ −4.71251 * 10⁻⁶ 1.17646 * 10⁻⁷ 2.37242 * 10⁻⁸ 2−1.50023 * 10⁻²    2.74410 * 10⁻⁴    5.36367 * 10⁻⁶ −2.32047 * 10⁻⁶  2.12657 * 10⁻⁷ 0.00000 0.00000 4   4.01091 * 10⁻⁵  −4.37622 * 10⁻⁶   6.69159 * 10⁻⁷ −1.00549 * 10⁻⁷ 0.00000 0.00000 0.00000 6   6.07734 *10⁻⁷  −1.04643 * 10⁻⁷    5.49308 * 10⁻⁹ 0.00000 0.00000 0.00000 0.000008 −1.35263 * 10⁻⁸  −5.90017 * 10⁻¹⁰ 0.00000 0.00000 0.00000 0.000000.00000 10   4.77561 * 10⁻¹⁰ 0.00000 0.00000 0.00000 0.00000 0.000000.00000

[0062] TABLE 5 Example 1 . . . coefficient b_(ij) of phase functions pof the fourth surface (S4) j i 0 2 0   0.00000 −1.11792 * 10 2 −7.051040.00000

[0063] TABLE 6 Example 2 . . . coordinate data of the optical surfacesorigins of y vector of local local coordinate x vector of localcoordinate systems coordinate systems systems Surface X Y Z X Y Z X Y, ZS1 Flat 0.00 53.81 0.00 0.0000 −1.0000 0.0000 1.0000 0.0000 S2Free-formed 0.00 50.69 0.00 0.0000 −0.6428 0.7660 1.0000 0.0000 S3Free-formed 0.00 49.81 −5.00 0.0000 0.6428 −0.7660 1.0000 0.0000 S4 Flat0.00 44.81 −5.00 0.0000 −1.0000 0.0000 1.0000 0.0000

[0064] TABLE 7 Example 2 . . . configuration data a_(ij) of the secondsurface (S2) j i 0 1 2 3 4 5 6 0 0.00000 0.00000   1.38024 * 10⁻²−1.47351 * 10⁻⁴ 3.10369 * 10⁻⁵ −3.32729 * 10⁻⁶ 1.59651 * 10⁻⁶ 2  1.80246 * 10⁻⁴   6.06191 * 10⁻⁴ −6.48022 * 10⁻⁵   6.80711 * 10⁻⁶8.51042 * 10⁻⁷ 0.00000 0.00000 4 −1.30531 * 10⁻⁵   2.28048 * 10⁻⁶−2.85520 * 10⁻⁷ −8.77701 * 10⁻⁷ 0.00000 0.00000 0.00000 6   7.65995 *10⁻⁷ −8.42971 * 10⁻⁷   9.43848 * 10⁻⁸ 0.00000 0.00000 0.00000 0.00000 8−8.00712 * 10⁻⁸   3.38222 * 10⁻⁸ 0.00000 0.00000 0.00000 0.00000 0.0000010   1.54079 * 10⁻⁹ 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

[0065] TABLE 8 Example 2 . . . configuration data a_(ij) of the thirdsurface (S3) j i 0 1 2 3 4 5 6 0 0.00000 0.00000 −1.51186 * 10⁻²  1.53456 * 10⁻⁴ −8.43217 * 10⁻⁶ 1.27153 * 10⁻⁷ −7.00577 * 10⁻¹⁰ 2−1.57061 * 10⁻²    3.07590 * 10⁻⁴ −2.83914 * 10⁻⁶   3.67962 * 10⁻⁷−5.73957 * 10⁻⁸ 0.00000 0.00000 4   1.63537 * 10⁻⁵    1.15762 * 10⁻⁶−5.24957 * 10⁻⁷ −1.00004 * 10⁻⁷ 0.00000 0.00000 0.00000 6 −1.03208 *10⁻⁶  −4.47100 * 10⁻⁸   5.50944 * 10⁻⁸ 0.00000 0.00000 0.00000 0.00000 8  5.19566 * 10⁻⁸  −5.45437 * 10⁻⁹ 0.00000 0.00000 0.00000 0.000000.00000 10 −2.86103 * 10⁻¹⁰ 0.00000 0.00000 0.00000 0.00000 0.000000.00000

[0066] TABLE 9 Example 3 . . . coordinate data of the optical surfacesorigins of y vector of local local coordinate x vector of localcoordinate systems coordinate systems systems Surface X Y Z X Y Z X Y, ZS1 axisymmetric 0.00 53.81 −4.00 0.0000 −1.0000 0.0000 1.0000 0.0000diffracting surface S2 free-formed 0.00 50.69 0.00 0.0000 −0.6428 0.76601.0000 0.0000 S3 free-formed 0.00 49.81 −5.00 0.0000 0.6428 −0.76601.0000 0.0000 S4 diffracting 0.00 44.81 −5.00 0.0000 −1.0000 0.00001.0000 0.0000 surface

[0067] TABLE 10 Example 3 . . . configuration data a_(i) of the firstsurface (S1) i 0 1 1.18993 −2.97483 * 10⁻¹

[0068] TABLE 11 Example 3 . . . configuration data a_(ij) of the secondsurface (S2) j i 0 1 2 3 4 5 6 0 0.00000 0.00000   1.99020 * 10⁻²−3.92558 * 10⁻⁴   1.80850 * 10⁻⁴ −1.190256 * 10⁻⁵ −2.40408 * 10⁻⁶ 2  7.41897 * 10⁻³   6.80752 * 10⁻³ −2.71699 * 10⁻⁴   6.30746 * 10⁻⁵−1.11421 * 10⁻⁵ 0.00000 0.00000 4   2.58916 * 10⁻⁴ −4.49741 * 10⁻⁵  2.57710 * 10⁻⁵ −3.10481 * 10⁻⁶ 0.00000 0.00000 0.00000 6   3.52931 *10⁻⁶   1.77167 * 10⁻⁶ −1.80463 * 10⁻⁶ 0.00000 0.00000 0.00000 0.00000 8−3.27275 * 10⁻⁷ −1.50344 * 10⁻⁷ 0.00000 0.00000 0.00000 0.00000 0.0000010 −4.69916 * 10⁻⁹ 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

[0069] TABLE 12 Example 3 . . . configuration data a_(ij) of the thirdsurface (S3) j i 0 1 2 3 4 5 6 0 0.00000 0.00000 −1.51542 * 10⁻²1.41647 * 10⁻⁴ −1.85750 * 10⁻⁵ 4.65418 * 10⁻⁷ 2.27003 * 10⁻⁷ 2−1.99559 * 10⁻²    1.89728 * 10⁻³ −2.45037 * 10⁻⁵ 2.77256 * 10⁻⁷−2.09185 * 10⁻⁷ 0.00000 0.00000 4   1.51702 * 10⁻⁵    1.16567 * 10⁻⁵−9.02800 * 10⁻⁷ 1.38173 * 10⁻⁸ 0.00000 0.00000 0.00000 6 −1.72045 *10⁻⁶  −1.97829 * 10⁻⁷   8.96208 * 10⁻⁸ 0.00000 0.00000 0.00000 0.00000 8  6.73980 * 10⁻⁸    4.44134 * 10⁻⁹ 0.00000 0.00000 0.00000 0.000000.00000 10 −5.17474 * 10⁻¹⁰ 0.00000 0.00000 0.00000 0.00000 0.000000.00000

[0070] TABLE 13 Example 3 . . . configuration data a_(ij) of the fourthsurface (S4) j i 1 0 −1.62128 * 10⁻¹

[0071] TABLE 14 Example 3 . . . coefficient b_(i) of phase functions pof the first surface (S1) i 0 1 1.18993 −2.97483 * 10⁻¹

[0072] TABLE 15 Example 3 . . . coefficient b_(ij) of phase functions pof the fourth surface (S4) j i 1 2 0 1.09000 * 10² −5.00000

[0073] TABLE 16 Example 4 . . . coordinate data of the optical surfacesorigins of y vector of local local coordinate x vector of localcoordinate systems coordinate systems systems Surface X Y Z X Y Z X Y, ZS1 free-formed 0.00 30.00 −4.00 0.0000 0.0000 1.0000 1.0000 0.0000 S2free-formed 0.00 30.00 0.00 0.0000 0.7071 0.7071 1.0000 0.0000 S3axisymmetric 0.00 25.00 0.00 0.0000 −1.0000 0.0000 1.0000 0.0000diffracting surface

[0074] TABLE 17 Example 4 . . . configuration data a_(ij) of the firstsurface (S1) j i 0 1 2 0 0.00000 0.00000 6.98477 * 10⁻³ 2 −3.52755 *10⁻³ −6.48250 * 10⁻³ 0.00000

[0075] TABLE 18 Example 4 . . . configuration data a_(ij) of the secondsurface (S2) j i 0 1 2 3 4 5 6 0 0.00000 0.00000 −8.21053 * 10⁻³−4.83841 * 10⁻⁵   1.77165 * 10⁻⁶ 3.10338 * 10⁻⁸ −4.35176 * 10⁻⁸ 2−1.44870 * 10⁻²  −1.16735 * 10⁻³  −6.83222 * 10⁻⁵ −5.86029 * 10⁻⁶−3.28807 * 10⁻⁷ 0.00000 0.00000 4   1.63693 * 10⁻⁵    3.86115 * 10⁻⁶   7.36155 * 10⁻⁷   8.97919 * 10⁻⁸ 0.00000 0.00000 0.00000 6 −6.98102 *10⁻⁸  −5.42497 * 10⁻⁸  −3.24219 * 10⁻⁹ 0.00000 0.00000 0.00000 0.00000 8−3.63095 * 10⁻¹⁰   9.69579 * 10⁻¹⁰ 0.00000 0.00000 0.00000 0.000000.00000 10   4.14544 * 10⁻¹¹ 0.00000 0.00000 0.00000 0.00000 0.000000.00000

[0076] TABLE 19 Example 4 . . . coefficient b_(i) of phase functions pof the third surface (S3) i 2 −1.00000 * 10

What is claimed is:
 1. A light scanning apparatus comprising: a lightsource emitting a light beam; a deflector for deflecting an incidentlight beam in a main scanning direction; a light source optical systemconstituted by one optical element made of resin and having at least onereflecting surface having no symmetry axis of rotation and twotransmitting surface, said light source optical system shaping the lightbeam emitted from the light source into a substantially parallel beam inthe main scanning direction, and condensing the light beam in a vicinityof a deflecting surface of the deflector in a sub scanning direction;and a scanning optical system again condensing the light beam deflectedby the deflector.
 2. The light scanning apparatus as claimed in claim 1,wherein the light is totally reflected on the reflecting surface.
 3. Thelight scanning apparatus as claimed in claim 1, wherein the opticalelement has two reflecting surface.
 4. The light scanning apparatus asclaimed in claim 1, wherein at least one surface of the transmittingsurface is a diffracting surface.
 5. The light scanning apparatus asclaimed in claim 1, wherein the refracting power is canceled out by thediffracting power at the diffracting surface.
 6. The light scanningapparatus as claimed in claim 1, wherein the principal ray incident onthe diffracting surface passes in a straight line.
 7. The light scanningapparatus as claimed in claim 1, wherein both of the two transmittingsurfaces are diffracting surfaces and the diffraction angles of the twodiffracting surfaces change opposite to each other when the wave lengthchanges.
 8. A laser scanner comprising: a laser light source emitting alaser beam; a deflector deflecting an incident laser beam in a mainscanning direction; a light source optical system constituted by oneoptical element made of resin and having: a first transmitting surfaceon which the laser beam emitted from the laser light source is incident;at least one reflecting surface reflecting the laser beam incident onthe first transmitting surface, and having no symmetry axis of rotation;and a second transmitting surface from which the laser beam reflected bythe reflecting surface exits, said light source optical system shapingthe laser beam emitted from the laser light source into a substantiallyparallel beam in the main scanning direction, and condensing the laserbeam in a vicinity of a deflecting surface of the deflector in a subscanning direction; and a scanning optical system again condensing thelaser beam deflected by the deflector.
 9. The laser scanner as claimedin claim 8, wherein the laser light is totally reflected on thereflecting surface.
 10. The laser scanner as claimed in claim 8, whereinthe optical element has two reflecting surface.
 11. The laser scanner asclaimed in claim 8, wherein at least one surface of the transmittingsurface is a diffracting surface.
 12. The laser scanner as claimed inclaim 8, wherein the refracting power is canceled out by the diffractingpower at the diffracting surface.
 13. The laser scanner as claimed inclaim 8, wherein the principal ray incident on the diffracting surfacepasses in a straight line.
 14. The laser scanner as claimed in claim 8,wherein both of the two transmitting surfaces are diffracting surfacesand the diffraction angles of the two diffracting surfaces changeopposite to each other when the wave length changes.
 15. A laser scannerscanning a laser light on the photosensitive member of an image formingapparatus, said laser scanner comprising: a laser light source emittinga laser beam; a deflector deflecting an incident laser beam in a mainscanning direction; a light source optical system shaping the laser beamemitted from the laser light source into a substantially parallel beamin the main scanning direction, and condensing the laser beam in avicinity of a deflecting surface of the deflector in a sub scanningdirection; and a scanning optical system again condensing the laser beamdeflected by the deflector, wherein said light source optical systemconstituted by one optical element made of resin and having: a firsttransmitting surface on which the laser beam emitted from the laserlight source is incident; at least one reflecting surface reflecting thelaser beam incident on the first transmitting surface, and having nosymmetry axis of rotation; and a second transmitting surface from whichthe laser beam reflected by the reflecting surface exits.
 16. The laserscanner as claimed in claim 15, wherein the laser light is totallyreflected on the reflecting surface.
 17. The laser scanner as claimed inclaim 15, wherein at least one surface of the transmitting surface is adiffracting surface.
 18. The laser scanner as claimed in claim 15,wherein both of the two transmitting surfaces are diffracting surfacesand the diffraction angles of the two diffracting surfaces changeopposite to each other when the wave length changes.